Mint Virginia won't shut down

I posted here because I don't know if this is a hardware or software problem, and it's affecting a laptop. My machine has suddenly stopped shutting down properly. When I give it the command to shut down the screen goes from my usual background to black, and asks me for a password. If I enter it the machine then reboots. It will only do this, and I have to hard down it in order to shut it down. This is the second go 'round for this issue. The first time (about 2 weeks ago) it cured itself before I could figure out what was going on. Any thoughts on how to diagnose this?